Determining Olive Oil Quality with Gas Chromatography

A joint study between the Department of Plant Molecular Biotechnology of the National Institute of Genetic Engineering and Biotechnology (NIGEB), in Tehran, Iran, and the Department of Plant Genetics and Production at the College of Agriculture of Shiraz University, in Shiraz, Iran conducted an RNA-seq analysis on Olea europaea cultivar (cv) Mari and Olea europaea cv Shengeh, two Iranian olive cultivars (defined as a kind of cultivated plant selected for its desired traits, and which retains those traits when propagated [1]), for the first time, aiming to uncover the genetic and molecular mechanisms that influence oil quality. Additionally, the study seeks to identify differentially expressed genes and pathways linked to oil quality. Gas chromatography (GC) confirmed a significant difference in fatty acid composition between the two cultivars. An article based on this analysis was recently published in the journal Frontiers in Plant Science (2).

Olive oil’s well-balanced composition of fatty acids enhances its stability against oxidation, which makes it a valuable addition to a healthy diet. Its high level of unsaturated fatty acids and the low level of saturated fatty have allowed olive oil to have positive effects on inhibiting cancers and cardiovascular diseases. (3,4). Oleic acid is the main fatty acid found in olive oil, making up 55-83% of the total fatty acid content, while other fatty acids present include linoleic acid (2.50-21.00%), palmitic acid (7.5-20.00%), stearic acid (0.5-5.00%), and linolenic acid (<1%) (5). High levels of oleic acid and low levels of linoleic, linolenic, and palmitic acids are what makes olive oil offer superior nutritional benefits and to have their quality regarded as suitable (6). The quantities of these fatty acids in olive oil can vary depending on the source’s genotype, the environment in which they were grown, and the interaction between the two (7,8). Iran is renowned in the Middle East as a region with great diversity of olive germplasm (9).

In terms of oil quality, Mari and Shengeh were selected as two extreme Iranian olive cultivars. Three trees were selected as replicates in each cultivar, with ten fruits randomly chosen around the canopy from each replicate for GC analyses, with their oil extracted from the mesocarp tissues of the fruits by the cold pressing method. The GC results revealed that the main fatty acids in olive oil in both cultivars were palmitic acid (16:00), palmitoleic acid (16:1), stearic acid (18:0), oleic acid (18:1), linoleic acid (18:2), and alpha-linolenic acid (18:3), and also indicated that palmitic acid was the dominant saturated fatty acid and oleic acid was the dominant unsaturated fatty acid in both selected cultivars. The percentage of oleic acid in Mari’s mesocarp (78.48%) was significantly higher than in Shengeh (48.04%), and the percentage of linoleic acid in Mari (4.76%) was significantly lower than in Shengeh (26.69%). Based on the percentage of oleic acid and linoleic acid, the oleic acid to linoleic acid (O/L) ratio in Mari was 16.49% and 1.80% in Shengeh. Furthermore,the study findings indicated that the ratio of total unsaturated fatty acids to total saturated fatty acids is a key factor affecting the oxidative stability of olive oil (2).

Analysis of differential gene expression identified 2775 genes with statistically significant expression patterns, with 33% of differentially expressed genes (DEGs) up-regulated and 67% down-regulated. Pathway enrichment analysis revealed key genes involved in fatty acid biosynthesis and oil quality pathways. The authors state that this highlights the importance of pathways such as glycolysis, galactose metabolism, fatty acid biosynthesis, and glycerolipid metabolism in the biosynthesis of fatty acids (2).

Olive oil in vintage bottles. © fahrwasser - stock.adobe.com

References

1. Cultivar definition. Wikipedia. https://en.wikipedia.org/wiki/Cultivar (accessed 2024-10-18)

2. Asadi, A.; Tavakol, E.; Shariati, V.; Hosseini Mazinani, M. Unraveling the Genetic Basis of Oil Quality in Olives: A Comparative Transcriptome Analysis. Front. Plant Sci. 2024, 15, 1467102. DOI: 10.3389/fpls.2024.1467102

3. Yubero-Serrano, E. M.; Lopez-Moreno, J.; Gomez-Delgado, F.; Lopez-Miranda, J.; Extra Virgin Olive Oil: More Than a Healthy Fat. Eur. J. Clin. Nutr. 2019, 72, 8–17. DOI: 10.1038/s41430-018-0304-x

4. Jimenez-Lopez, C.; Carpena, M.; Lourenço-Lopes, C.; Gallardo-Gomez, M. M.; Lorenzo, J.; Barba, F. J., et al. Bioactive Compounds and Quality of Extra Virgin Olive Oil. Foods 2020, 9, 1014. DOI: 10.3390/foods9081014

5. International Olive Council (IOC0. International Trade Standard Applying to Olive Oils and Olive-Pomace Oils. 2019. https://www.internationaloliveoil.org/what-we-do/chemistry-standardisation-unit/standards-and-methods. (accessed by authors of [1] 2020-06-27).

6. Salimonti, A.; Carbone, F.; Romano, E.; Pellegrino, M.; Benincasa, C.; Micali, S. et al. Association Study of the 5′UTR Intron of the FAD2-2 Gene with Oleic and Linoleic Acid Content in Olea europaea L. Front. Plant Sci. 2020, 11. DOI: 10.3389/fpls.2020.00066

7. Mele, M. A.; Islam, M. Z.; Kang, H. M.; Giuffrè, A. M. Pre-and Post-Harvest Factors and their Impact on Oil Composition and Quality of Olive Fruit. Emirates J. Food Agric. 2018, 30, 592–603. DOI: 10.9755/ejfa.2018.v30.i7.1742

8. Navas-López, J. F.; Cano, J.; de la Rosa, R.; Velasco, L.; León, L. Genotype by Environment Interaction for Oil Quality Components in Olive Tree. Eur. J. Agron. 2020, 119, 126115. DOI: 10.1016/j.eja.2020.126115

9. Hosseini-Mazinani, M.; Mariotti, R.; Torkzaban, B.; Sheikh-Hassani, M.; Ataei, S.; Cultrera, N. G. M. et al. High Genetic Diversity Detected in Olives Beyond the Boundaries of the Mediterranean Sea. PloS One 2014, 9, e93146. DOI: 10.1371/journal.pone.0093146